Recently, tools based on scatterometry have been developed for process control in photolithography, and particularly for micro-lithography, which provide for high accuracy and repeatability, faster measurement, smaller volume and lower cost. Scatterometry is a method by which the signature, or optical response, of a periodic structure is measured. The signature may be obtained by measuring the optical properties of a structure, such as reflectance, ellipsometric or other parameters, as a function of one or more light parameters, such as the angle of incidence, polarization or wavelength. Thus, term “signature” may refer to an optical response of the structure to predetermined incident light. Due to the periodicity of the structure, it is possible to theoretically calculate the signature of a given sample using exact models thereof, such as in accordance with Rigorous Couple Wave Theory (RCWT), where a measured signature is correlated with theoretically calculated signatures that fit the structure's parameters. This fitting method suffers from such drawbacks as long calculation time, in-adequacy to real-time calculations, and the need for detailed knowledge about the structure, such as optical constants, that is required as input to the model. The problem of long calculation time is usually overcome by preparing a library of pre-calculated signatures. This procedure, however, requires a long setup time. The detailed knowledge about the structure, in many cases, also requires preliminary setup processes, such as material characterization. Yet another problem is the complicated, sometimes indirect relation between the process parameters, such as focus and exposure, and the profile parameters, where attempting to control the process by modifying process parameters based on profile information is difficult to implement. These problems impede the application of scatterometry-based systems as a production tool, specifically for integrated monitoring that require a fast feedback for process control. Additionally, since the accuracy of any measurement depends on the precision of the measuring device, any error of the measuring device decreases the reliability of such measuring techniques. Therefore, there is a need for an improved and efficient method for photolithographic process control.